This is by far my favorite "Complete _____'s Handbook." Nearly everything in here is essential if you want to have a real thief, and not just a bunch of stats. The kits are some of the best I've seen. Other Handbooks have outlandish or just downright worthless kits (for instance, take that Ranger kit that turns you into a tree!). In this one, however, every kit is unique, and yet believable and fun to play. For a long time, this handbook was the only information available on Thieves' Guilds. If that's what you're after, however, I'd recommend Den of Theieves, which fleshes it out even more and provides all sorts of adventure possibilities. Round this book out with new equipment and magic items, and you have a classic.